How polar bears survive the Arctic

A study of the genes of polar bears reveals how quickly they evolved to handle the extremes of life in the high Arctic, and why, and how they cope with being profoundly obese. A comparison between polar and brown bears has found that the former is a much younger species than previously believed, having diverged from brown bears less than 500,000 years ago to spend life on sea ice. There, the bears subsist on a blubber-rich diet of marine mammals that would result in cardiovascular diseases in other species. The relatively short time that has passed in its evolution and how it evolved was what interested the scientists.

Unlike other bears, fat comprises up to half the weight of a polar bear. “For polar bears, profound obesity is a benign state,” said Lorenzen. “We wanted to understand how they are able to cope with that. The life of a polar bear revolves around fat. Nursing cubs rely on milk that can be up to 30 per cent fat, and adults eat primarily blubber of marine mammal prey. Polar bears have large fat deposits under their skin and, because they essentially live in a polar desert and don’t have access to fresh water for most of the year, rely on metabolic water, which is a by product of the breakdown of fat.”

The genome analysis comes at a time when the polar bear population worldwide, estimated at between 20,000 and 25,000, is declining and its Arctic sea ice habitat is rapidly disappearing. As the northern latitudes warm, the polar bear’s distant cousin, the brown or grizzly bear is moving farther north and occasionally interbreeding with the polar bear to produce hybrids that have been called ‘pizzlies’. This is the possibly the same process that led to the emergence of polar bears in the first place.

The bears’ ability to interbreed is a result of a very close relationship, Nielsen said, which is one-tenth the evolutionary distance between chimpanzees and humans. “It’s really surprising that the divergence time is so short. All the unique adaptations polar bears have to the Arctic environment must have evolved in a very short amount of time.”

These adaptations include not only a change from brown to white fur and development of a sleeker body, but big physiological and metabolic changes as well. The genome comparison revealed that over several hundred thousand years, natural selection drove major changes in genes related to fat transport in the blood and fatty acid metabolism. One of the most strongly selected genes is APOB, which in mammals encodes the main protein in LDL (low density lipoprotein), known widely as “bad” cholesterol. Changes or mutations in this gene reflect the critical nature of fat in the polar bear diet and the animals’ need to deal with high blood levels of glucose and triglycerides, in particular cholesterol, which would be dangerous in humans.

What drove the evolution of polar bears is unclear, though the split from brown bears coincided with a particularly warm 50,000-year interglacial period known as Marine Isotope Stage 11. Environmental shifts following climate changes could have encouraged brown bears to extend their range much farther north. When the warm interlude ended and a glacial cold period set in, a pocket of brown bears may have become isolated and forced to adapt rapidly to new conditions.

There is potential for the polar bear research also to have applications in the study of human’s lifestyles. “Polar bears have adapted genetically to a high fat diet that many people now impose on themselves,” said Nielsen. “If we learn a bit about the genes that allows them to deal with that, perhaps that will give us tools to modulate human physiology down the line.”